The present invention relates generally to equipment and methods utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a metal to metal seal.
Wellhead plugs which utilize metal to metal seals are well known in the art. The metal to metal seals are typically secured in wellheads using anchoring devices, with the seals being installed in seal bores of the wellheads. Typically, the seals are only effective in sealing against pressure differentials applied in one direction, for example, from below the wellhead.
However, there are many circumstances in which it would be desirable for a metal to metal seal to be able to seal against pressure differentials applied alternately from above and below the wellhead. For example, it may be desired to perform a pressure test in a riser above the wellhead, in which case the wellhead plug should be capable of containing the pressure in the riser above the wellhead. Of course, there are many other circumstances in which a bidirectional metal to metal seal would be desirable.
Another limitation of current metal to metal seals is that they are unable to satisfactorily regulate contact pressures between the seals and the bores against which they seal. In a typical metal to metal seal, an increase in pressure differential applied from one axial direction tends to expand the seal and increase the contact pressure at a rate which quickly leads to yielding of the seal and/or bore material. The seal may be made more rigid to limit the rate of contact pressure increase, but this solution causes other problems, such as requiring the use of exceptionally tight machining tolerances, etc.
Yet another problem occurs with metal to metal seals which are installed in grooves. Such a seal presents at least two leak paths. One leak path is between the seal and the bore, and the other leak path is between the seal and the groove.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a plug is provided which utilizes a metal to metal seal for well plugging applications. Associated methods are also provided.
In one aspect of the invention, a plugging system is provided which includes a metal seal circumferentially contacting a bore and a piston attached to the seal. The piston biases the seal in the direction of a pressure differential applied across the piston.
When the pressure differential is applied in one axial direction, tensile axial stress is applied to the seal by the piston. This tensile stress may act to radially inwardly bias the seal. The pressure differential may also act to radially outwardly bias the seal due to circumferential tensile stress. The stresses in the seal may be adjusted by appropriately configuring the plug structure, so that a desired contact pressure is achieved when a particular pressure differential is applied in the axial direction.
When the pressure differential is applied in an opposite axial direction, compressive axial stress is applied to the seal by the piston. This compressive stress may act to radially outwardly bias the seal. The pressure differential may also act to radially inwardly bias the seal due to circumferential compressive stress. The stresses in the seal may be adjusted by appropriately configuring the plug structure, so that a desired contact pressure is achieved when a particular pressure differential is applied in the opposite axial direction.
In another aspect of the invention, radially inwardly and outwardly biasing of the seal may be regulated or limited by the piston portion of the plug. For example, an increased rigidity of the piston will better Emit the inwardly and outwardly biasing of the seal.
In still another aspect of the invention, the plug may include a substantially hollow spherical structure having opposite open and closed ends, with the seal being positioned between the ends. The open end admits pressure into the interior of the structure, and the closed end prevents pressure transmission therethrough. The seal is disposed on a circumferentially continuous portion of the structure between the ends.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings.